Ceramic compositions, physical vapor deposition targets and methods of forming ceramic compositions

ABSTRACT

The invention includes a method for forming a ceramic composition. Materials comprising lead, zirconium, titanium and bismuth are combined together to form a mixture. At least one of the materials is provided in the mixture as a nanophase powder. The mixture is then densified to form the ceramic composition. The invention also includes a method for forming a dense ferroelectric ceramic composition. Lead, zirconium, titanium and bismuth are combined together to form a mixture. The mixture is then densified to form a ferroelectric ceramic composition having a density of greater than or equal to 95% of a theoretical maximum density for the composition. A predominate portion of the composition has a grain size of less than or equal to about 500 nanometers. The invention also includes a ferroelectric ceramic composition comprising lead, zirconium, titanium and bismuth. Such composition has a density of greater than or equal to 95% of a theoretical maximum density for the composition, and a predominate portion of the composition has a grain size of less than or equal to about 500 nanometers.

TECHNICAL FIELD

The invention pertains to methods of forming ceramic compositions andferroelectric ceramic compositions. The invention also pertains toparticular ceramic compositions and ferroelectric ceramic compositions,and to physical vapor deposition targets.

BACKGROUND OF THE INVENTION

Ceramic compositions comprising lead, zirconate, and titanate (i.e,so-called PZT compositions) have numerous uses in electrical devices.For instance, ferroelectric ceramic compositions comprising PZT can beused as piezoelectric transducers, electroactuators, capacitors, andelectro-optic devices. Additionally, ferroelectric PZT materials can beused as thin film deposition targets in forming portions of non-volatileferroelectric random access memories (FeRAMs), pyroelectric devices andmicro-electromechanical systems (MEMS) by physical vapor deposition.

PZT can be modified for particular applications. For instance, lanthanummodified PZT (referred to as PLZT) is reported to have exceptionalproperties for utilization in FeRAMs.

PZT ceramics have a number of advantages for utilization in electricaldevices. For instance, PZT ceramics can have a high electromechanicalcoupling coefficient, which can render them particularly useful aspiezoelectric transducers. Additionally, PZT ceramics can have a highpyroelectric coefficient, which can make them particularly suitable asinfrared detectors. Further, PZT materials can have a high dielectricconstant, which can render them particularly useful as dielectricmaterials in capacitor constructions. Also, PZT ceramics can havesuperior electrooptic properties, which can render them suitable aselectro-optic switches.

A difficulty in utilizing PZT ceramics is in fabrication of suitable PZTmaterials. Specifically, a number of the PZT applications describedabove utilize relatively large-dimension PZT ceramics. It can bedifficult to control processing conditions to insure high density andgood reproducibility throughout a large PZT ceramic. An exemplaryapplication wherein a relatively large PZT ceramic is utilized isphysical vapor deposition. Specifically, a PZT ceramic can be utilizedas a sputtering target for physical vapor deposition during formation ofsmall-scale circuits, such as, for example, FeRAMs. It is desired thatthe target be comprised of homogeneous grains having dimensions of 1micrometer or less (with smaller grain sizes being more desirable thanlarger grain sizes), have a density approaching maximum theoreticaldensity, and be a single phase perovskite structure. Homogeneity andfine grain size of the target are desired in order that the target canbe utilized to deposit films with appropriate chemical composition,uniformity, and preferred orientation at a rapid deposition rate.

A conventional method of forming PZT materials is to densify a mixturecomprising lead, zirconium, and titanium. The lead, zirconium andtitanium can be in the form of, for example, PbZrO₃, PbTiO₃, and/orPb(Zr, Ti)O₃. One method of densifying such mixture is throughsintering, another method is through hot-pressing, and yet another isthrough sinter-forging.

A typical sintering method is pressureless (i.e., occurs at atmosphericpressure, and for purposes of interpreting the claims, sintering is tobe assumed to be pressureless unless stated otherwise), and is asfollows. First, a mixture comprising lead, zirconium and titanium iscold-pressed into a so-called green compact. The mixture can furthercomprise a binder, such as, for example, polyvinyl acetate. The term“cold-pressing” refers to pressing occurring at or below about 30° C.,and typically at a pressure of about 10,000 pounds/in². Once formed, thecold-pressed pellet is subjected to a temperature of at least 1100° C.,and typically from 1200° C. to 1400° C., to sinter the pellet. The hightemperatures of the sintering process utilize a large amount of heatenergy. Such heat energy can deteriorate and erode the interior of asintering furnace. Accordingly, maintenance costs of sintering furnacescan become expensive. Further, the high temperatures utilized for thesintering process can result in the relatively volatile material PbObeing released from the ceramic. The release of PbO removes lead. Theloss of lead can alter a composition of the ceramic, and can complicatereproduction of PZT ceramics in sequential sintering processes.

A method which has been utilized to compensate for the loss of lead isto add additional lead to a PZT composition prior to sintering. However,the additional lead can cause its own problems in the form ofinhomogeneous lead distribution, and difficulty in controlling leadconcentration and lead-site vacancies within a PZT ceramic formed from amaterial comprising excess lead.

As mentioned above, hot-pressing can be utilized instead of sinteringfor densifying a PZT. Hot-pressing typically comprises placing a mixtureof lead, zirconium, and titanium in a press and compressing the mixtureto a pressure of less than 10,000 pounds/in² and typically less than6,000 pounds/in². During the pressing, a temperature of the material istypically maintained at more than 700° C. An advantage of utilizinghot-pressing instead of sintering can be that hot-pressing frequently isdone at lower temperatures than sintering. The lower temperatures ofhot-pressing can avoid some of the above-discussed problems associatedwith sintering. However, a difficulty with hot-pressing is thatrelatively large equipment is used to press even a small amount ofceramic material. The expenses associated with forming large-scalehot-pressing facilities for pressing large amounts of PZT materialsreduces the economic feasibility of hot-pressing processes. Further, thethroughput in production of large-scale PZT ceramic materials usinghot-pressing is lower than can be accomplished with sintering.Accordingly, sintering can be more attractive than hot-pressing forcommercial-scale PZT production.

The last of the above-identified methods of densifying PZT materials,sinter-forging, involves subjecting a green compact (a green compact isreferred to above in describing the sinter process) to hot-pressing.Sinter-forging methods can be difficult to commercialize for reasonssimilar to those discussed above regarding hot-pressing methods.

Because of the commercialization potential of sintering processes,several methods have been utilized in an attempt to improve sinteringprocesses. Among such methods is the addition of sintering aids, suchas, for example, metal oxides and fluoride compounds to PZT ceramiccompositions. Such additions can reduce sintering temperatures, and thuslower manufacturing costs, while enabling reasonable control of the leadcontent in resulting PZT ceramics. In particular applications,utilization of additives has been shown to reduce sintering temperaturesdown to 950° C. for PZT compositions which contain lower valencesubstituents (e.g., Fe, Mn). However, additives have not been foundwhich can successfully reduce a sintering temperature below 1050° C. forbulk PZT compositions comprising lanthanum. Further, no process (eithersintering, or hot-pressing) has produced a PZT material with a densitygreater than or equal to 95% of a theoretical maximum density of thematerial (about 7.9 gm/cm³) while keeping a predominate grain sizewithin the PZT material to less than 500 nanometers. It would bedesirable to develop dense PZT materials with small grain sizes for manyapplications, including, for example, physical vapor deposition targetapplications.

SUMMARY OF THE INVENTION

In one aspect, the invention includes a method for forming a ceramiccomposition. Materials comprising lead, zirconium, titanium and bismuthare combined together to form a mixture. At least one of the materialsis provided in the mixture as a nanophase powder. The mixture is thendensified to form the ceramic composition.

In another aspect, the invention encompasses a method for forming adense ferroelectric ceramic composition. Lead, zirconium, titanium andbismuth are combined together to form a mixture. The mixture is thendensified to form a ferroelectric ceramic composition having a densityof greater than or equal to 95% of a theoretical maximum density of thecomposition. A predominate portion of the composition has a grain sizeof less than or equal to about 500 nanometers.

In yet another aspect, the invention encompasses a method for forming aceramic composition. Materials comprising lead, zirconium, titanium andantimony are combined together to form a mixture, with at least one ofthe materials being provided in the mixture as a nanophase powder. Themixture is then densified to form the ceramic composition.

In yet another aspect, the invention encompasses a ferroelectric ceramiccomposition comprising lead, zirconium, titanium and bismuth. Suchcomposition has a density of greater than or equal to 95% of atheoretical maximum density for the composition, and a predominateportion of the composition has a grain size of less than or equal toabout 500 nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a flow chart description of a method encompassed by thepresent invention.

FIG. 2 is a view of a physical vapor deposition apparatus encompassed bythe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

One aspect of the invention is a method of forming a ferroelectricceramic composition in accordance with a process outlined in the flowdiagram of FIG. 1. At initial step 10 of such flow diagram, lead,zirconium, titanium and bismuth are combined together to form a mixture.The lead, zirconium and titanium can be in the form of PbZrO₃, PbTiO₃and/or Pb(Ti,Zr)O₃, and the bismuth can be in the form of bismuth oxide(Bi₂O₃). If PbZrO₃ and PbTiO₃ are utilized, such can be combined in theratio (1−x)PbZrO₃:(x)PbTiO₃, wherein x is from 0.1 to 0.9. The bismuthoxide preferably comprises from about 0.1 to about 10 weight percent ofthe mixture, and more preferably comprises from about 0.5 to about 3weight percent of the mixture. Additional materials, such as, forexample, lanthanum, can be included in the mixture. Further, materialscan be included which substitute for lead at some of the sites withinPZT. For instance, a cationic form of at least one element selected fromthe group consisting of potassium, sodium, cesium, calcium, strontium,barium, yttrium or lanthanide elements can be included in the mixture.Also, elements can be included which substitute for some of thezirconium or titanium at sites within the PZT. For instance, a cationicform of iron, chromium, hafnium, tin, manganese, niobium, tantalum ortungsten can be included in the mixture. Further, one or both ofantimony oxide and vanadium oxide can be included in the mixture. Inalternative embodiments of the invention, the mixture can consistessentially of lead, zirconium, titanium, bismuth and oxygen. In yetother alternative embodiments of the invention, the mixture can consistessentially of lead, zirconium, titanium, bismuth, lanthanum and oxygen.

It is preferred that at least some of the materials in the mixture areprovided in the form of nanophase powders, with the term “nanophasepowder” referring to a material having a predominate portion thereofwith a maximum particle size of less than or equal to 500 nanometers,and preferably less than or equal to 100 nanometers. Although only apredominate portion (i.e., greater than 50%) having a particle maximumsize of less than 500 nanometers is enough for the powder to fit withinthe above definition of a “nanophase powder”, the powder can consistessentially of particles having a maximum dimension of less than orequal to 500 nanometers, and can further consist essentially ofparticles having a maximum dimension of less than or equal to 100nanometers. It is noted that some agglomeration of particles can occurto form agglomerates having maximum dimensions larger than 500nanometers. As long as the agglomerated particles remain distinguishablefrom one another and have maximum sizes of less than 500 nanometers, theagglomerated particles are still nanophase particles.

Step 10 of FIG. 1 can be accomplished by, for example, mixing theabove-described components in a wet carrier such as, for example,isopropyl alcohol. In a particular embodiment, the components are mixedin isopropyl alcohol by magnetic stirring for about five hours.

The mixed components are in the form of a wet mixture. Such mixture ispassed through a sieve (step 12 of FIG. 1) to break apart agglomeratedparticles, and dried at about 80° C. The sieve can have a mesh size of,for example, from about 100 mesh to about 200 mesh. The dried materialis cold-pressed (step 14 of FIG. 1) to form a so-called “green compact”.The green compact is sintered (step 16 of FIG. 1).

An advantage of utilizing nanophase powders and bismuth in the steplabeled “10” of FIG. 1, is that such combination can lower apressureless sintering temperature (i.e., a sintering temperature atatmospheric pressure) relative to compositions which lack the nanophasepowders and bismuth, and can enable higher density materials to beformed than can be formed without the nanophase powders and bismuth. Forinstance, sintering of PZT comprising one weight percent bismuth oxideat 900° C. for a time of about 6 hours leads to a ceramic materialcomprising 100% of the theoretical density for such material (about 7.9gm/cm³), and sintering of PLZT comprising one weight percent bismuthoxide at 900° C. for a time of about 6 hours leads to a ceramic materialhaving 100% of the maximum theoretical density for such material (about7.9 gm/cm³, with the actual density being determined using, for example,the Archimedes method). Additionally, even lower sintering temperaturescan be used and appreciable densities achieved. For instance, sinteringtemperatures can be lowered to 850° C., and after 15 hours at suchtemperatures PZT comprising one weight percent bismuth oxide achieves99% of the theoretical maximum density, while PLZT containing one weightpercent bismuth oxide achieves 98% of the theoretical maximum density.

The high densities achieved with sintering of the above-describedcompositions are substantially higher than densities achieved withconventional compositions. Further, grain sizes of high densitycompositions of the present invention can remain small. Specifically, apredominate portion of a composition of the present invention can have amaximum grain size of from about 100 nanometers to about 5 microns, andfrequently a predominate portion of the composition will have a maximumgrain size of less than about 500 nanometers. In particularcompositions, a predominate portion will have a grain size of less than200 nanometers, less than 150 nanometers, or less than 100 nanometers.In other compositions, a maximum grain size of the entirety of thecomposition will be less than or equal to about 500 nanometers; in yetother compositions, a maximum grain size of the entirety of thecomposition will be less than or equal to about 200 nanometers; in yetother compositions, a maximum grain size of the entirety of thecomposition will be less than or equal to about 150 nanometers; and inyet other compositions, a maximum grain size of the entirety of thecomposition will be less than or equal to about 100 nanometers.

High density compositions of the present invention can also be achievedat sintering temperatures less than or equal to about 800° C. Forinstance, PZT comprising one weight percent bismuth oxide, and PLZTcomprising one weight percent bismuth oxide were found to achievedensities of 98% and 95%, respectively, when sintered at 800° C. for 24hours.

The effect of bismuth oxide concentration in various PZT mixtures candetermine an optimum bismuth oxide concentration for forming ceramiccompositions. It is found that samples comprising at least 0.3 weightpercent bismuth oxide show a significant decrease in sinteringtemperature relative to samples comprising less bismuth oxide. Further,it is found that samples comprising more than three weight percentbismuth oxide show less decrease in sintering temperature as compared tosamples containing 1 weight percent bismuth oxide. Also, it is foundthat in samples in which 10 weight percent bismuth oxide is utilized, adensity of only 93% is reached at a sintering temperature of 1150° C.

Bismuth oxide can have some effect on properties of PZT other thandensity and grain size. For instance, electrical resistivity is found toincrease with higher bismuth concentrations in PZT materials. However,the materials remain ferroelectric when bismuth is included, asdetermined by polarization hysteresis of PZT materials comprisingbismuth therein. Remnant polarization appears to be dependent on bismuthoxide content, as well as on a sintering condition utilized to form aceramic material. High amounts of bismuth oxide appear to reducepolarization.

Bismuth oxide can also influence a dielectric response of a ceramicmaterial comprising PZT and bismuth oxide. Specifically, slightly higherdielectric constant and lower loss factors are obtained with materialscomprising bismuth oxide relative to materials that do not comprisebismuth oxide.

It is noted that the above-discussed ceramic materials comprisingbismuth oxide remain as a single perovskite phase, as confirmed usingx-ray diffraction.

Sintering temperature can influence dielectric responses in PLZTcomprising one weight percent bismuth oxide. It is found that higherdielectric constants are generally obtained at lower sinteringtemperatures, while dielectric loss factors are relatively low. Forexample, a dielectric constant and loss factor are found to be 903 and0.026, respectively, for a material sintered at 900° C. for six hours.In addition, electrical resistivity of the material is found to be about3×10¹¹ ohms-cm. Such properties are actually better than those of PLZTwithout additives for applications in which the PLZT is utilized as adielectric material.

Ferroelectric properties of PLZT comprising one weight percent bismuthoxide indicate that polarization switching cannot be fully achieved forceramics sintered below 900° C. The polarization becomes more easilyswitched with increasing sintering temperature. If the material issintered at 1150° C., complete polarization switching can beaccomplished. However, the remnant polarization is 20 μC/cm², which is alittle lower than that of pure PLZT. This may result from the smallergrain size of the ceramic material that results when bismuth oxide isincluded, relative to the grain size when bismuth oxide is not included.An average grain size of PLZT comprising bismuth oxide is determined tobe about 900 nanometers when the material is sintered at 1100° C., andabout 200 nanometers when the material is sintered at 800° C. Such grainsize is smaller than that which would be obtained when sintering PLZTlacking bismuth oxide under conventional conditions. In a preferredcomposition of PZT, or PLZT, the material will have a predominateportion with a maximum grain size of less than 500 nanometers afterdensification, and in particular preferred compositions, a maximum grainsize throughout the composition will be less than 500 nanometers. Inother compositions, the PZT or PLZT material will have a predominateportion with a maximum grain size of less than 200 nanometers afterdensification, and in yet other compositions, the PZT or PLZT materialwill have a predominate portion with a maximum grain size of less than150 nanometers after densification.

In light of the above-discussed grain sizes determined for PZT (whichincludes PLZT) compositions comprising bismuth and formed with nanophasepowders, it can be preferred that densification comprise a sinteringtemperature of less than about 1100° C., and more preferred that thedensification comprise a sintering temperature of less than about 850°C.

Although the above-described methodologies for densifying materialscomprised sintering, it is to be understood that materials of thepresent invention can also be densified by hot pressing orsinter-forging.

Regardless of the densification procedure utilized, the materialsresulting from the densification can comprise, for example, a generalformula Pb(Zr_((1−x))Ti_(x))O₃, wherein x is from 0 to 1. Alternatively,if lanthanum is included to form PLZT, the compositions can comprise,for example, (Pb_((1−3y/2))La_(y))(Zr, Ti)O₃, wherein y is from 0.01 to0.5, and preferably from 0.02 to 0.30. If cationic forms of at least oneelement selected from the group consisting of potassium, sodium, cesium,calcium, strontium, barium, yttrium or lanthanide elements are includedto substitute for some of the lead in the resulting PZT mixture, atleast some of the mixture can be in the form of (Pb,M)(Zr_((1−x))Ti_(x))O₃, wherein x is from 0 to 1 and “M” represents thecationic form of the at least one element. Further, if a second cationis provided comprising a cationic form of at least one element selectedfrom the group consisting of iron, chromium, hafnium, tin, manganese,niobium, tantalum or tungsten to substitute for at least some of thetitanium or zirconium, at least some of the material in the mixture canhave the form (Pb, M)(Zr_((1−x−z))Ti_(x)B_(z))O₃, with (x+z) being from0 to 1, “B” being one of the cations, and “M” being another of thecations.

It is found that inclusion of nanophase powders and bismuth oxide inmixtures of the present invention not only result in lowered sinteringtemperatures for PZT with tetragonal structures (i.e., PZT compositionswith high relative amounts of Ti), but can also be utilized forrhombohedral and orthorhombic structured PZT compositions (i.e.,compositions comprising a high zirconium/titanium ratio). For instance,the sintering behavior of lead zirconate evidences that a fully densestructure is obtained after sintering at 1050° C. for two hours.Polarization measurements of such structure indicate that the structureis antiferroelectric, with a dielectric constant and loss factor of 166and 0.002, respectively. It is found, however, that sintering at thelower temperature does not result in a full density. This may be causedby the coarser grain sizes of a PZO precursor powder in comparison withthose for PZT. In fact, less than 80% of the theoretical maximum densityis obtained after sintering at 1050° C. using micron-sized leadzirconate powders. Such could indicate that nanometer size powders areplaying a significant role in enhancing densification.

Although the above-described ceramic composition were formed byproviding bismuth oxide into a mixture comprising lead, zirconium andtitanium, methods of the present invention can be utilized with otheradditives. For instance, in another embodiment of the invention,antimony oxide (Sb₂O₅) and/or vanadium oxide (V₂O₅) is combined withmaterials comprising lead, zirconium and titanium to form a ceramicmaterial. Preferably, at least some of the combined components areprovided as nanophase powders. It is found, however, that neitherantimony oxide nor vanadium oxide works as well as bismuth oxide forlowering a sintering temperature of a PZT material.

Compositions of the present invention can be utilized in numerousapplications in conventional PZT materials are being used. One such sapplication is as a sputtering target for formation of micro-electronicdevices. FIG. 2 illustrates an apparatus 20 utilizing a physical vapordeposition target 22 encompassing compositions of the present invention.Apparatus 20 has a semiconductive material wafer 24 therein and spacedfrom target 22, and a backing plate 26 retaining target 22 in a properorientation relative to wafer 24. Sputtered material 28 is shown beingtransferred from target 22 to wafer 24 to form a film of the sputteredmaterial over a surface of the wafer.

Compositions of the present invention can be preferred relative to priorart PZT compositions for forming physical vapor deposition targets, suchas, for example, sputtering targets, as the ceramic materials formed bythe present invention can be dense, uniform throughout theircomposition, and can comprise small grain sizes.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A physical vapor deposition target, comprising: abacking plate; and a ferroelectric ceramic composition joined to thebacking plate, the composition comprising lead, zirconium, titanium andbismuth; a density of the composition being greater than or equal to 95%of a theoretical maximum density for the composition; a predominateportion of the composition having a grain size of less than or equal toabout 500 nanometers.
 2. The target of claim 1 wherein a maximum grainsize of the composition is less than or equal to about 500 nanometers.3. The target of claim 1 wherein a predominate portion of thecomposition has a grain size less than or equal to about 200 nanometers.4. The target of claim 1 wherein a predominate portion of thecomposition has a grain size less than or equal to about 150 nanometers.5. The target of claim 1 wherein a maximum grain size of the compositionis less than or equal to about 150 nanometers.
 6. The target of claim 1wherein the composition comprises a density of at least 98% of thetheoretical maximum density.
 7. The target of claim 1 wherein thecomposition comprises a density of at least 99% of the theoreticalmaximum density.